Conventionally, a driving force adjustment apparatus is known in which a differential apparatus interposed between the left and right wheels of a vehicle and a planetary gear mechanism and a motor are combined such that distribution of the driving force (torque distribution) between the left and right wheels can be changed. In such a driving force adjustment apparatus as just described, the motor rotates passively in response to the difference between rotational speeds of the left and right wheels upon turning of the vehicle to absorb the rotational speed difference. Further, as the motor operates, the driving force difference between the left and right wheels increases or decreases to change the distribution of the driving force between the left and right wheels. On the other hand, if rotation of the motor is restrained, then the differential action is limited and the traction performance is improved (e.g. JP 2007-177915 A).
Since an existing driving force adjustment apparatus is structured such that a planetary gear mechanism is interposed on a rotational shaft of the left and right wheels, it has a subject that the dimension thereof in the vehicle widthwise direction is liable to increase and downsizing is difficult. Especially, such a driving force adjustment apparatus as disclosed in JP 2007-177915 A has a complicated structure in which two planetary gear mechanisms are disposed on an axle. Therefore, the size of the entire apparatus increases in the vehicle widthwise direction, which gives rise to degradation of the mountability. Further, since the length of the axle decreases, there is a demerit that this gives rise to degradation of the ground performance of the vehicle.
Further, in the existing driving force adjustment apparatus, since the motor rotates normally in response to the rotational speed difference between the left and right wheels, in a travelling state in which driving force distribution is not required, there is the possibility that power loss by unnecessary rotation of the motor may occur, resulting in degradation of the operation performance.
Further, the existing driving force adjustment has only a function for adjusting the driving force between the left and right wheels and, in order to obtain driving force for driving a vehicle using a motor, it is necessary to separately provide a motor for traveling and a reduction mechanism. Therefore, there is the possibility that further degradation of the mountability may be caused.